1. Field of the Invention
The present invention relates to a method and an apparatus for examination utilizing a nuclear magnetic resonance (hereinafter referred to as an "NMR") phenomenon to determine a particular atomic nucleus distribution within the body of a subject being examined from the outside of the body, and more particularly to an NMR examination method and apparatus suitable for use in medical diagnostic systems.
The principles of an NMR technique will first be described prior to the description of the prior art and the present invention.
An atomic nucleus is composed of protons and neutrons which rotate as a whole with a nuclear spin angular momentum I. FIGS. 1A and 1B of the accompanying drawings show an atomic nucleus of hydrogen (.sup.1 H). As shown in FIG. 1A, the hydrogen atomic nucleus has a single proton P making rotation expressed by a spin quantum number 1/2. Since the proton P has a positive electric charge e.sup.+ as illustrated in FIG. 1B, a magnetic moment .mu. is produced as the atomic nucleus rotates. Thus, each hydrogen atomic nucleus can be regarded as one small magnet.
FIGS. 2A and 2B schematically illustrate how atomic nuclei as small magnets are oriented. For a ferromagnetic material such as iron, atomic nuclei as small magnets are directed in alignment as shown in FIG. 2A, and the material as a whole exhibits a magnetization. For elements such as hydrogen, however, atomic nuclei as small magnets are oriented randomly in various directions, that is, they have their magnetic moments directed in various directions, as illustrated in FIG. 2B, and the material has no magnetization as a whole.
When such a material is placed in a static magnetic field H.sub.0 in the direction of Z, each atomic nucleus is aligned in the direction of the magnetic field H.sub.0, that is, the nuclear energy level is quantized in the direction of Z.
FIG. 3A shows the manner in which hydrogen atomic nuclei are oriented in a static magnetic field. Since the spin quantum number of a hydron atomic nucleus is 1/2, it has two energy levels of -1/2 and +1/2 as shown in FIG. 3B. The energy gap .DELTA.E between the two energy levels can be expressed by the following equation (1): EQU .DELTA.E=.delta.hH.sub.0 ( 1)
where
.delta.: Gyromagnetic ratio, PA1 h=h/2.pi. PA1 h: Planck's constant.
Because each atomic nucleus undergoes the force expressed by EQU .mu..times.H.sub.0
due to the static magnetic field H.sub.0, the atomic nucleus precesses about the Z-axis at an angular velocity given by the equation (2) EQU .omega.=.gamma.H.sub.0 (Larmor angular velocity) (2)
When this system is subjected to an electromagnetic wave (normally known as a radio wave) having a frequency corresponding to the angular velocity .omega., there occurs a resonance and the atomic nucleus absorbs an energy equivalent to the energy gap .DELTA.E expressed by the equation (1) and is shifted to a higher energy level. Even where several kinds of atomic nuclei having nuclear spin angular momentums are present in a mixed state, the different atomic nuclei have their respective Gyromagnetic ratios and the frequencies at which they resonate are varied, so that the resonance of a particular kind of atomic nuclei can be picked up. By measuring the intensity of the picked-up resonance, the amount of atomic nuclei present can be determined. The atomic nuclei which have shifted to the higher energy level will return to the lower energy level upon elapse of an interval of time determined by a time constant known as a relaxation time after the resonance.
There are two types of relaxation times; a spin-lattice relaxation time (longitudinal relaxation time) T.sub.1 and a spin-spin relaxation time (transverse relaxation time) T.sub.2. Data on a material distribution can be obtained by observing these relaxation times. Since spins generally are substantially fixed in a position on a crystal lattice for solid materials, it is relatively easy for the spins to interact with each other. Therefore, solid materials have a short relaxation time T.sub.2, and the energy created by nuclear magnetic resonance is first spread into the spin system and then into the lattic system. Accordingly, the relaxation time T.sub.1 is much longer than the relaxation time T.sub.2. In liquids, however, molecules freely move and thus the ease with which energy exchange is liable to occur remains substantially the same for a spin-spin system and for a spin-molecule (lattice) system. Consequently, the relaxation times T.sub.1, T.sub.2 are substantially equal to each other. The relaxation time T.sub.1 is a time constant dependent on how compound molecules are bonded, and it has been known that normal tissues and malignant tumors have widely different relaxation times T.sub.1.
It is possible to effect the same measurements with other atomic nuclei having nuclear spin angular momentums than hydrogen atomic nuclei (.sup.1 H). Examples of such other atomic nuclei include phosphorus atomic nuclei (.sup.31 P), carbon atomic nuclei (.sup.13 C), sodium atomic nuclei (.sup.23 Na), fluorine atomic nuclei (.sup.19 F), and oxygen atomic nuclei (.sup.17 O).
Accordingly, the amount of a certain kind of atomic nuclei and their relaxation times can be measured by NMR. The body of a subject can be examined in various ways by obtaining various items of chemical information on particular atomic nuclei in materials in the body.
2. Description of the Prior Art
Known examination apparatus utilizing an NMR phenomenon operate on the same principles as an X-ray CT by exciting protons in an imaginary section of the body of a subject being examined, finding NMR signals for respective projections in many direction in the body of the subject, and determining the intensity of the NMR signal in each position of the subject body through a reconstruction process.
FIG. 4 of the accompanying drawings shows the waveforms of signals illustrative of an examination process in the prior NMR examination apparatus.
The body of a subject is first placed in a Z-gradient magnetic field Gz.sup.+ as shown at (B) in FIG. 4, and subjected to RF pulses (90.degree. pulses) each having a narrow frequency spectrum as shown at (A) in FIG. 4. At this time, protons are excited only in a plane characterized by a Larmor angular velocity .omega.=.gamma.(H.sub.0 +.DELTA.Gz). If a magnetization M is shown on a revolving coordinate system as shown in FIG. 5A which revolves at an angular velocity .omega., the magnetization M is oriented in the direction of y'-axis which is 90.degree. angularly spaced from the direction of z'-axis. Then, x-gradient magnetic field Gx and y-gradient magnetic field Gy are applied to the subject body to generate a two-dimensional magnetic field, thereby detecting an NMR signal as shown at (E) in FIG. 4. Since the magnetization M is progressively dispersed in the directions of the arrows in the planes x', y' as shown in FIG. 5B due to irregularities of the magnetic field, the NMR signal is gradually reduced in intensity until it finally is eliminated upon elapse of the time .tau. as illustrated at (E) in FIG. 4. The NMR signal thus obtained is subjected to a Fourier transform, thus providing a projection perpendicular to the gradient magnetic field which is a combination of the x-gradient magnetic field Gx and the y-gradient magnetic field Gy.
The next sequence of operation is repeated in the same manner upon elapse of a predetermined time .tau.'. The gradient magnetic fields Gx, Gy are slightly changed in each sequence. Thus, the NMR signal corresponding to each projection can be found in many directions in the body of the subject being examined.
With the conventional apparatus, the time period .tau. in FIG. 4 required for the NMR signal to disappear ranges from 10 to 20 mS. The prescribed time .tau.' required until the next sequence is started is about 1 sec. due to the relaxation time T.sub.1. Therefore, assuming that one section of the subject body is to be reconstructed with 128 projections, it will take at least 2 minutes for the particular body section to be thoroughly measured.